Positron lifetime has been measured as a function of temperature in Sidoped GaAs single crystals subjected to various heat treatments. Defects produced by these heat treatments trap positrons. In all the GaAs samples containing defects positron lifetime was found to decrease with temperature in the range from 375 K to 16 K. The decrease is explained as due to the decrease in the trapping rate. The trapping rate is mainly controlled by the diffusion of the positron to the trap. The diffusion constant is determined mainly by the scattering from charged Si impurities.

The structure of multilayered carbon tubules has been investigated by electron microscopy and X-ray diffraction. The structure of tubules is characterized by disorder in the stacking of cylindrical graphene sheets. Raman scattering measurements have been carried out in tubules and compared with graphite. The observed features in the Raman spectra in tubules can be understood in terms of the influence of disorder. The additional Raman modes predicted for single layer carbon tubules have not been observed.

Nickel substitution at the cobalt site in Na_0.75CoO₂ induces an upturn in the resistivity on lowering the temperature, with the metal-to-insulator transition temperature (T_MIT) increasing with the Ni content. Low temperature far infrared measurements on polycrystalline samples of Na_0.75CoO₂ and Na_0.75Co_0.95Ni_0.05O₂, the latter having T_MIT ∼ 175 K, have been carried out. Dramatic changes in the Na mode frequencies, and relative intensities of the out-of-plane modes corresponding to the two Na sites are observed, coincident with the MIT in Na_0.75Co_0.95Ni_0.05O₂. It is argued that these changes are associated with a charge ordering of the CoO₂ layer, associated with the metal-insulator transition.